Topic: CB750 Four K3 - Wannabe Restor'd

[Rostuff]

Hi all, so this is my first post here after a lot of lurking. The idea of starting this project has been going around my head for many years, I'd say 5 since my beauty got bored of carrying me around and decided to smoke me up. The culprit is indeed in the top left cilinder, I knew it suffered of low pressure issues but since I didn't use the bike a lot at that time I didn't think tearing down the motor was really necessary. Well, it was, but let's talk about now.

First off, here she is





As you can see there are lots of minor issues going around, and some of them you won't even see in these pics.

The one and only issue I wanna start with is fixing the engine. I believe what caused it to fail was either the gasket (but which one?) or one/some or the bearings, but I wanna check the piston's rings too. The idea is to change them all and hope that the rest of the engine is still ok. Just for reference the bike was running (could only go in 1st or 2nd gear at very low rpms) when I put her to sleep 5 years ago, so rods and pistons should be fine. I guess.

If everything goes fine I'll proceed fixing the rest of the bike with all those "little things" that are missing or just too old.

But the real question is: I've never ever done such a thing in my life. I've been around motorbikes for a long time, I know how things works and I'm always happy when I can try to mod or tweak something and do my own maintainance to my (working) motorbikes. I also used to do all kind of nasty stuff when I was a kid to my Vespa's and other 50cc engines, but that was literally a joke compared to this one here. Is it doable for a sort of newbie like me? The problem is not much in tearing it down and changing the parts rather than in the closing it up and doing all the tweaking, registrations, tolerances, valves, shafts.....

Is there anything that I should consider before even starting the tearing down process? As for parts to be changed, I have a list and I'll need to ask you details on that since I'm not even sure which are the part numbers I need to order (mainly talking about the bearings here but will talk about that later) but is there anything I should change no matter what once I tear down a motor like this one?

All right, thanks for all the ideas and tips you may have, I hope to update this one soon.

[dhall57]

Welcome from North Carolina. I'm not a engine rebuilder. But their are many here that have done just that and all could give you tips and advice I'm sure. But first off get yourself a shop manual on these sohc4 CB750's. I think one can be downloaded right off this forum. Try going to the home page under resources and look for Manuals and Tech Documents. Also forum member Hondaman(he's called this for a good reason) as a good book out also.

[Rostuff]

Yup I have downloaded the heck out of the internet and have all the manuals. I was really tempted by the Hondaman book but I'll wait for that. Besides there are so many threads here to learn from but it's still in the cart.

Now, couple of "serious" problems encountered. First one has been fixed but I had to sacrifice the oil filter's carter bolt. After a lot of WD-40 love and several attempts the good and righteous way, I had to go hard on him and when I say hard I mean this hard





Second one, which is still to be solved, is about the carb block. I removed the airbox and all the clamps from the boots/inlets whatever they're called and I thought the carb block would kinda slip out of the way with a sort of oscillating-and-pulling-backwards movement to the whole block. Problem is that those boots are stiff like 40 years old boots and the oscillating movement doesn't sort much of an effect. I did it bare hands because I didn't want to risk applying too much force and damaging a carb or two but ended up nearly slitting my palm :s

Is there any other way to apply some force to the carb block without risking to break anything? I don't want to cut them boots so that's not an option.

[DennyK]

I found that using a short piece of 1x2 lumber as a pry bar gave me some additional leverage and did not hurt the fins.  You can protect the fins with some additional rags as well.  I had success prying on 1 and 4.  Also just grabbing them with your hands, wear gloves, and moving them up and down can help.  As a practical matter, you are going to have to replace those boots at some point in your restoration.  Good Luck.

[Stev-o]

You'll have to get phisicall to remove the carbs.  The airbox is off, right?

Gloves and feet help!

[Rostuff]

Things moved a bit onwards. Yesterday I finally reached my first and most important goal (or "endpoint #1") which was basically removing the engine. To be completely honest the first and main goal was to have a look at the pistons, and that hasn't been succeded yet, but more of that later.

So, thanks to your advices I was finally able to remove the carb block. I actually used the wooden log/pry bar method on carb 1 and 4, but it has been a much harder fight than to remove the engine. Didn't expect that but that's also because when it was time to go for the hard work the mighty power of stone-age-engineering came at me in the form of "handles of love". Let me show you this revolutionary new mothod I'm going to patent that will allow you to remove the engine block in no longer than 10 minutes without having to mess around a beached motorbike rattling on its side while you try to pull the chassis out of the engine instead of the other way around. Ah, and this is a pure "soloist method", totally family&neighbors free (priceless isn't it?).

First things first: you need a handle. I think I bought these as equipment for a hike in the mountains, but you can make it with a piece of rope or Walmart it anyway.





Than you need one of these old jacks for your old car. You'll put it right under the oil carter to rotate the engine kinda counterclockwise



in order to be able to refit the upper rear stud and use it as a hook for the aforementioned handles. Just like that



Then of course you must hook it up on the the other side of the stud



And there you go: handles of love!





But that's how you'll use it for the first couple of pulls and rotations, then you'll need to turn it into this



and here comes the real fun. Now, I don't mean you won't have to sweat it, but as I said it won't take you more than 5-7 mins if you do it right and by right I mean you make a step at a time and don't lose time and efforts without understanding which part is blocking what and make small and (kinda) precise movements to disengage everything that comes in the way. After said few minutes, this is what you'll get:



Bingo!

[Rostuff]

Now, the previous was a sort of a (pretentious) educational post, now I need to make one to ask for some advice. I've been really surprised that I was able to remove all the screws without having some real hard time on them. Of course they're all to be replaced and impossible to be reused but I didn't have to drill or cut a single one. For sure the WD-40 gods have been busy on them but as I said that came quite as a surprise.

Anyway, I'm at this point now



How do I reach "stage 2" aka the pistons chamber? Can I remove the valve block without having to undo every single one of them? If it was up to me I'd unscrew all the 12 nuts on the top and try to remove the shaft with the valves attached to their own "leverages" (sorry dunno the name of those) then proceed with the 8 more nuts right below the shaft and hope that's enough.

But I could use some of your advice at this point. Tia.

[calj737]

Before proceeding, do yourself an enormous favor and:

 Go to www.lulu.com and search for "My CB750 Book" to find it, then use the checkout code there.

http://couponfollow.com/site/lulu.com
http://www.offers.com/lulu/
or you can try your own search for "Lulu Publishing Coupon" and maybe find even better deals!

You need to buy Mark Parris' book and use it as a reference along with the shop manuals.

[70CB750]

+1 on the book.

As of now, get a box,  draw a plan on it.  You will be punching holes in it for each bolt, closed box lid workes the best.

You want to back off valve adjustment tappets first, they are on the end of each rocker arm. 

Take out and put in proper place on the box 4 bolts that hold rocker shafts, they are half way between rocker arms.

Remove rocker shafts and rocker arms and put them back together, place on the box.

Next, take off the cam shaft holders, place them properly.

Take off the cam shaft sprocket bolts and get the cam shaft out.  Place a stick under the chain to keep it up.  If you drop it in it can always be fished out later. But you dont want to turn the engine over with the chain fallen in.

Cam towers next, again, put it aside the way it was, every bolt needs to go back to the same place on reassembly.

Now you are ready to take the head off.

Did I miss anything?

[70CB750]

Important:   

Under cam towers are two oil jets, make sure you take them out and keep safe, they are hard to find.

Always turn the engine from the left side, that side can take it, do not use the nut over the ignition plate, only if you really have to, it can be bent easily.

[Rostuff]

Before proceeding, do yourself an enormous favor and:

 Go to www.lulu.com and search for "My CB750 Book" to find it, then use the checkout code there.

http://couponfollow.com/site/lulu.com
http://www.offers.com/lulu/
or you can try your own search for "Lulu Publishing Coupon" and maybe find even better deals!

You need to buy Mark Parris' book and use it as a reference along with the shop manuals.

Yep thanks, the book is definitely something I'm considering, except for a reason: I still don't know how this project is gonna end. That's why I need to look into the pistons first and see what caused the initial issue and what happened after I kept on running for about 5-10 Km almost without oil. If the situation is too compromised I'll have to leave it and sell the bike for parts or something. If it's still possible to restore it properly then I'll surely get the book and study much more than I did so far.

[Rostuff]

+1 on the book.

As of now, get a box,  draw a plan on it.  You will be punching holes in it for each bolt, closed box lid workes the best.

You want to back off valve adjustment tappets first, they are on the end of each rocker arm. 

Take out and put in proper place on the box 4 bolts that hold rocker shafts, they are half way between rocker arms.

Remove rocker shafts and rocker arms and put them back together, place on the box.

Next, take off the cam shaft holders, place them properly.

Take off the cam shaft sprocket bolts and get the cam shaft out.  Place a stick under the chain to keep it up.  If you drop it in it can always be fished out later. But you dont want to turn the engine over with the chain fallen in.

Cam towers next, again, put it aside the way it was, every bolt needs to go back to the same place on reassembly.

Now you are ready to take the head off.

Did I miss anything?

Pretty darn good informations there. I should be able to do that without "special tools" like the valve/cotter/springs/brackets removal thingy (sorry again, I'll learn all these technical words about engine parts in english once I'm really set to restore this thing) but I'm not sure I understood completely the last part about cam towers and where or what are those oil jets, but one step at a time.

[SOHC4 Cafe Racer Fan]

They are not kidding about the book. It is invaluable.

[70CB750]

Valve spring compressor can be rented from Autozone and other places, it works for motorcycle engine too.

Anyway, here is an oil flow diagram.  Oil goes from the oil pump to the crankshaft and to the head, jets limit the volume of oil that gets delivered to the head.

Jets are on the intake side, on top of cylinders, like this:



Once you take it apart you will understand, good luck

[Rostuff]

Had a couple of hours today to remove the cam shaft and loosen up the head bolts. And this is what I found under it



Dunno the name of those but they're clearly pressure control valves of some sort, and one was missing or maybe.....



..... just playing peekaboo. I have no idea if it could be the main cause of the problem, meaning where the air came into the combustion chamber from or just the consequence of some abnormally high-pressure inside of it. If you do, let me know.

By the way, this are some closeups of the oil jets. Here too, there is clearly something not going perfectly well since one has a kinda misplaced and worn out washer/oring and the other one simply has none.





Anyway, now I'm in this situation



Everything removed, spark plugs too and all the bolts loosen up. I'd say I'm ready to remove the head but fella doesn't look like moving a single bit (tried with the might of my mallet). Am I missing something? Do I have to go hard on it, like with a real hammer and some wooden log?

[70CB750]

The rubber puck you missing is there to keep oil from coming out.  Common problem. 

Now you need to take off all the fasteners that hold the head on:



But frankly get the book or at least shop manual and read it first.

[Rostuff]

Agreed, my bad. The shop manual is pretty clear about how to do things.



And yes, I'm that lazy 

[calj737]

And yes, I'm that lazy 

Lazy gets you killed when riding a motorcycle. Lazy makes things break when working on a bike. Lazy makes engines brake after you put them together.

Time to change your ways or your interests.

[Rostuff]

And yes, I'm that lazy 

Lazy gets you killed when riding a motorcycle. Lazy makes things break when working on a bike. Lazy makes engines brake after you put them together.

Time to change your ways or your interests.

Seriously? LoL thanks for sharing dude.

[calj737]

Yes, seriously, Dude.

I appreciate your wanting to "proceed slowly" before diving in and determining whether the motor can be rebuilt. But, to ignore practical counsel for referencing shop manuals and purchasing tomes of experience and insight in the name of "penny pinching" will cost you far more than you'll spend. If this motor is trashed and beyond repair of your budget, why not simply acquire another running, motor? Plenty out there. Thus, the manuals and books become investments like tools and have use and value beyond the moment.

Or, you can

"...go hard on it, like with a real hammer and some wooden log?"

and enjoy the spoils of your laziness.

[Rostuff]

Or maybe I'm just a chilldude who doesn't give a damn about looking like the super duper mechanic I'll never be and just wants to enjoy restoring the bike of his youth in his (scarce) freetime. How about that?

Anyway, in the proverbial meantime...... lazy chilldude finally got INTO the piston chamber and



Guess where is the problem?

Overall the situation is not as horrible as I thought. There are a couple of oxidated valves (1st and 4th cylinder) but the cylinders themselves look pretty good, the infamous 3rd obviously has some marks but I don't know if it needs to be rectified. I'll have to take some measurements but it may be allright. As for the piston's ring sets, they're all good except the 3rd one in which the lower 2 (the coupled thinner ones) look like melted inside their housing. This doesn't mean I won't change them all (of course I will). Rods and crankshaft look good to me so far.

The only "problem" is that I haven't been able to separate the head (the one with the valves) from the main body (the one with the cylinders) but I couldn't find any more screws or bolts and while pulling it just came loose all in one piece and.... well I just took it out in one piece lol

So the final verdict is that I'll give it a better look tomorrow but I think I'll stick around for some more time since it looks like lazy chilldude can make it! 

[70CB750]

Did you take out bolts 19 through 22?

[Rostuff]

Ehm.... actually that's a good question but.... nope. I was expecting them bolts to be exposed but it might be worth removing those little round things looking like pressure control seals which are instead probably only protecting the bolts underneath ahah

Learning something every day you see? 

[Rostuff]

And I've got me The Book.

[Stev-o]

Guess where is the problem?

Could it be the #2 super-low compression piston?